1. Field of the Invention
This invention relates generally to control systems for motors and, more particularly, to motor control circuits that keep the speed of a motor within a predetermined range.
2. Description of the Related Art
It often is important to control the speed of a motor with precision and reliability. Controlling the speed of a motor is especially important when the motor is used to operate machinery that could cause injury if the motor malfunctions. For example, if the motor is used to propel an amusement ride vehicle that carries passengers, a very specific motor speed profile must be followed with virtually no tolerance for error. In such an application, passengers can be injured if the motor speed increases during the ride such that the vehicle exceeds the specified speed. Conventional motor control systems can adequately limit motor speed, but are not sufficiently reliable to provide the virtually error-free matching of actual motor speed to the desired motor speed profile, such as ramping the speed up or down, that also are particularly important in the case of motors that propel ride vehicles.
Passengers can be injured if the actual motor speed does not reach the speed required in the profile, because the vehicle could have insufficient speed to safely negotiate the ride course. The actual motor speed also must follow the deceleration profile. For example, passengers can be injured if the motor allows the vehicle to speed up when the passengers are preparing to enter or exit the vehicle. Finally, the actual motor direction must propel the ride vehicle in the direction commanded by the profile, so that the vehicle is not moved in reverse when a forward motion is expected. The result of any of these improper motor operations can be catastrophic. Thus, the actual motor operation must match the motor speed profile. Many motor control systems cannot control the actual motor operation with the extreme reliability demanded for amusement park rides.
Various malfunctions can cause the actual motor speed to vary from what the commanded motor drive signals dictate, or can cause the drive signals to be different from the signals that should be provided. A motor control system is used to regulate the actual motor performance so that the actual speed matches the speed profile or at least so that the motor is shut down if the actual speed does not match. Effective motor control systems should include a means for obtaining reliable and accurate motor speed information that is easily integrated with the drive VFD. It is especially important to have accurate and reliable motor speed information if the motor is to be incorporated into a ride vehicle.
Conventionally, the actual speed of a motor is usually determined by attaching a tachometer to the shaft of the motor. A mechanical tachometer includes a mechanism that is rotated by the shaft and thereby indicates the motor's speed. The indicated speed is used to control the application of driving electrical power to the motor. The motor speed data is relatively easy to integrate with the control system, but the mechanical tachometer can become unreliable as various parts wear out. An electronic tachometer should provide greater reliability than a mechanical tachometer, and lends itself to integration with electronic motor drive systems. Such a tachometer, for example, can derive a speed signal by measuring the frequency of the motor current.
From the discussion above, it should be apparent that there is a need for a motor control system that can monitor and regulate motor performance with comparable accuracy and a higher degree of reliability than is achieved conventionally, and that can be much more easily incorporated into a motor drive system for control of the motor speed. The present invention satisfies this need.